Every rat (4 now) I've owned except one has died of cancer. Or at least, they died with cancer, I'm not a veterinarian. The one that didn't got the flu or something at 6 months and died of some respiratory issue.
They make fun affectionate pets if you can stomach the idea that they will die on you.
When they get a 'lump' it grows real quick. They make lovely, interactive pets, and very intelligent but they will break your heart and die too soon.